I have a 69 F-100, 390, and 3 on the tree. I was hot and heavy about changing over to a 4 speed to improve my rpms on the highway. I have been reading the forums and am thinking that aside from city driving being easier, it's not going to help on the top end. I run 2500 rpm at 55 mph. I think I have a 3:25 rear end (according to the tag).
Should I go to 4 speed? or change the rear end?
I need some recommendations guys.

 

Depends on the 4speed. Most of them from your era truck have a 1:1 4th gear just like your 3speed has, so there's no overdrive benefit there. Alot of them do have a granny low first gear and some of them may have a slightly lower second gear than your 3speed's first gear, so if low end grunt is what you're after, an NP435, T18, T98, or even T19 would do the trick. There are a couple 4speed trannys that have an overdrive 4th gear, but I don't know the names of them or what vehicles they came in. The granny gear 4speeds were used for about 25 years in pickups and some in Broncos too, so there are alot of those around.

2500 rpm at 55 isnt too high for a truck with no overdrive. Your 3.25 axle ratio is actually a fairly mild ratio. My '69 Bronco came with 3.50's. The size of the tire you are running makes a big difference in what gear ratio is best for your engine and intended use. The 3.50's in my Bronco with 31" tires and 302 engine with a 3speed that has a 3.03 (I think) first gear is a little higher (numerically lower) than I would like for offroading. It has no problem flying down the freeway though. If I put the stock size (29"?) tires on it, the gearing would feel alot lower (slower and more powerful) offroad and the top speed on the freeway would be lower too.

I would leave the trans and the rear end gears the way they are and change the tire size instead. Tires can be cheaper and certainly easier than swapping a trans or axle gears and if you dont like it you can always go back.

 

The final gear in both your 3 speed and a 4 speed will be 1:1 ratio. A overdrive transmission is needed to lower your RPMs on the hwy. You will like driving with the 4 speed though. I'm not sure if I would change your rear end (it's pretty high now), depends on the weight of the truck, your driving environment (hills?), etc., but it would lower your hwy RPMs. ...Terry

 

I mainly drive the truck in town and no towing. She's my baby. I have 28" tires and my dad is giving me some 33's that I'm thinking about getting. I'm not going to change the rear end, I don't want to stall at a stop sign.
Will a Dodge tranny work on a Ford? He also has a 5 speed off of a 70 or 71 van (150 I think). I know the NV 4500 will work but what about a plain old Dodge transmission.

 

I had a '77 F150 with three on the tree. Have to search my memory (sold it in '84) but seems I could go about 70 mph at 2500 rpm. Since you do not do much freeway cruising, I bet if you mount up those 33's you will really like how it performs and the lower freeway rpm. As for the rear end. Not sure you can interechange newer models, but I picked up an excellent rear end complete with brakes for my '95 a couple years ago for only $250 (open differential--limited slip twice as much).

 

Check through the threads for trans, I6 and newer econolines.

How much money do you want to spend?

A late model ZF has the lower 1st, 2,3,4 similar to 3 gear, 5th over.

Ain't cheap.

Ford 4speed OD's are a little weak but ok if you ain't gonna race. 1,2,3 is same as your 3 gear with the grand canyon between 2nd and 3rd.

Forget the Dodge/chevy/mercedes/yugo trannys--too expensive to adapt.

You could go really high dollar with a proper Ford Toploader WR or CR 4 gear with a Gear Vendors OD, or a Ranger OD----if you want to spend $4000 on the tranny area of the truck.

 

Debestuss, you may want to check your axle ratio. I'm doubting that it's a 3.25 ratio. Here's why: I ran your 2500rpms @ 55mph with 28" tires through the following equation and came up with 3.73 as the closest available ratio to give that engine rpm at that vehicle speed with that size tire.

GearRatio = (RPM x TireDia) / (MPH x 336).

Rearranging the equation to solve for RPM's, using your 3.25 ratio, your engine speed at 55mph should be about 2150rpm. Here's the equation rearranged to solve for rpms:

RPM = (MPH x GearRatio x 336) / TireDia.

It is possible that at some point in the past someone changed out the rear end for one with a different ratio.

You can test for gear ratio by lifting the rear axle so both wheels are off the ground and putting the trans in nuetral (with front wheels blocked, engine off) and mark the position of the tire and the driveshaft. If it's an open diff, which it probably is, turn the tire 2 full revolutions and count how many revolutions the driveshaft makes. If it's an open diff, the other tire should be spinning in the opposite direction while you're busy counting revolutions. That's the easiest way to tell what the actual ratio is without removing the 3rd member and counting the ring gear and pinion gear teeth.

The math just doesn't make sense for it to be a 3.25 ratio with the rpm and speed readings you gave and tire size you have. Do you have a tach on the truck or was that a guestimation of the engine speed?

 

I was ballparking it. You are exactly right, 2150 rpm at 55 mph. I have a tach. Normal, huh? Where do you guys get all those cool formulas?

 

Ah, glad that caper has been solved! Check out www.reiderracing.com , they have a few formulas in the back of their catalog which may also be on the website. Randy's Ring and Pinion also may have some: www.ring-pinion.com .